Inexpensive high-speed digital interfaces for relaying a digital signal have been provided in recent years. Therefore, for example, a radio communication system can be configured such that the radio communication system has a long distance between an amplifier device and a digital unit of a radio base station device as shown in FIG. 12.
Referring to FIG. 12, in a radio communication system, an amplifier device (transmitting/receiving power amplifier) provided near an antenna and a digital unit 4 of a radio base station device provided apart from the amplifier device are connected to each other via a digital signal line, and transmitting/receiving signals are transmitted as digital signals.
In this case, if a signal at a radio frequency is converted directly into a digital signal, an extremely high transmission rate is required. Therefore, a transmitting/receiving baseband signal is generally used as a relaying digital signal.
For a baseband signal, there are a method of using a digital signal sequence prior to a waveform shaping process such as filtering and a method of using a signal subjected to a waveform shaping process such as filtering or OFDM (orthogonal frequency division multiplexing). In the former method, a rate of an interface can be made equal to a transmission rate of transmitting/receiving signals. The former method is referred to as 1× sampling. In the latter method, a rate of an interface needs to be higher than a transmission rate of transmitting/receiving signals. The latter method is referred to as oversampling.
In the following description, a clock of a digital signal prior to oversampling (reference frequency) is referred to as a transmission clock, and a clock of a digital signal subjected to oversampling is referred to as a sampling clock.